Valve timing control device for an internal combustion engine

ABSTRACT

A valve timing control device for an internal combustion engine. The device comprises a variable valve timing mechanism capable of varying a valve overlap period; an increasing unit capable of increasing an amount of intake air in at least the idle condition; a determination unit for determining an optimal value of the valve overlap period in the current engine operating condition, on the basis of current engine speed, load, and temperature; a first controller for controlling the variable valve timing mechanism such that the valve overlap period becomes larger than the optimal value in an idle condition when the engine has not warmed up; and a second controller for controlling the increasing unit such that an amount of intake air is increased in an idle condition when the engine has not warmed up. Accordingly, in the idle condition when the engine has not warmed up, intake air is heated sufficiently, by the valve overlap period, to more than the optimal value and engine stall at such time is completely prevented by increasing the amount of intake air.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to a valve timing controldevice for an internal combustion engine, and in particular, to a valvetiming control device comprising a variable valve timing mechanism forcontrolling the valve overlap period.

2. Description of the Related Art

In an internal combustion engine, when the valve overlap period, duringwhich the intake valve and the exhaust valve are opened simultaneouslyat the end of an exhaust stroke, is made long, the trapping efficiencyand the scavenging efficiency increase so that good engine performancecan be obtained. On the other hand, in engine operating conditions underwhich the degree of opening of the throttle valve is relatively small sothat the negative pressure in an intake port becomes high, once thevalve overlap period is made long, the amount of back-flow exhaust gasinto the intake port becomes large so that combustion deteriorates.Accordingly, it is desirable that the valve overlap period is varied inaccordance with the engine operating condition. For this purpose, avariable valve timing mechanism capable of varying the valve overlapperiod has already been suggested.

According to the usual valve timing control device for controlling thevalve overlap period by means of the variable valve timing mechanism,during engine operating conditions under which the degree of opening thethrottle valve is relatively small, the valve overlap period is madeshort to prevent deterioration of combustion caused by a large amount ofback-flow exhaust gas. However, Japanese Unexamined Patent PublicationNos. 59-103910 and 2-298614 disclose a valve timing control device whichmakes the valve overlap period long during an idle condition when theengine has not warmed up. This is intended to positively utilize theback-flow of exhaust gas, as exhaust gas recirculation, by leading aportion of exhaust gas into the intake port to reduce the amount of NOxin the exhaust gas, by lowering the combustion temperature, and byheating the intake port to raise the intake air temperature.

However, during an engine idle speed (i.e. the idle condition) conditionwhen the engine has not warmed up, combustion is very unstable so thatthe engine can stall when the amount of back-flow exhaust gas isincreased. The valve timing control device disclosed in JP No. 2-298614mentioned above increases the amount of fuel and delays the ignitiontime to improve the combustion at this time. However, these cannotimprove the combustion sufficiently. As a result, engine stall can stilloccur.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a valvetiming control device for an internal combustion engine which is capableof heating the intake air sufficiently and completely preventing enginestall when the engine has not warmed up.

According to the present invention there is provided a valve timingcontrol device comprising a variable valve timing mechanism capable ofvarying the valve overlap period; increasing means capable of increasingthe amount of intake air in at least the idle condition; determinationmeans for determining the optimal value of the valve overlap period in acurrent engine operating condition, on the basis of current enginespeed, load, and temperature; first control means for controlling thevariable valve timing mechanism such that the valve overlap periodbecomes larger than the optimal value in an idle condition when theengine has not warmed up; and second control means for controlling theincreasing means such that the amount of intake air is increased in anidle condition when the engine has not warmed up. The present inventionwill be more fully understood from the description of the preferredembodiments of the invention set forth below, together with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic view of a valve timing control device according tothe present invention;

FIG. 2 is a main routine for controlling the valve overlap period via avariable valve timing mechanism by an electronic control unit;

FIG. 3 is a first sub-routine used as the sub-routine called by the mainroutine;

FIG. 4 is a second sub-routine used as the sub-routine called by themain routine;

FIG. 5 is a third sub-routine used as the sub-routine called by the mainroutine;

FIG. 6 is a fourth sub-routine used as the sub-routine called by themain routine;

FIG. 7 is a fifth sub-routine used as the sub-routine called by the mainroutine;

FIG. 8 is a sixth sub-routine used as the sub-routine called by the mainroutine;

FIG. 9 is a map, at a certain temperature of the cooling water, fordetermining the basic optimal value of the valve overlap period (AB)used in the main routine;

FIG. 10 is a map for determining the degree of opening of the ISC valve(IA) used in the main routine;

FIG. 11 is a map for determining the increased upper limit value of thevalve overlap period (dAmax) used in the first sub-routine;

FIG. 12 is a map for determining a coefficient (k2) used in the thirdsub-routine;

FIG. 13 is a map for determining an increased value of the valve overlapperiod (b) used in the fifth sub-routine;

FIG. 14 is a map for determining an increased value of the valve overlapperiod (c) used in the sixth sub-routine; and

FIG. 15 is a time chart showing the variation in the valve overlapperiod according to the sixth sub-routine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic view of a valve timing control device, accordingto the present invention. In this figure, reference numeral 1 designatesa combustion chamber and reference numeral 2 designates a piston. Anexhaust port 4 and an intake port 6 are connected to the combustionchamber 1 via an exhaust valve 3 and an intake valve 5, respectively. Afuel injector 7 is arranged downstream from a surge tank 6a in theintake port 6. A throttle valve 8 is arranged upstream of the surge tank6a in the intake port 6. Moreover, a bypass passage 9 which bypasses thethrottle valve 8 is provided. An idle speed control valve (ISC valve) 10which is driven by a step motor 10a is arranged in the bypass passage 9.

A pulley 11a is arranged on the end of a cam shaft 11 for the intakevalve 5. The pulley 11a is driven by the crank shaft (not shown) via abelt (not shown). On the other hand, a pulley 12a is arranged on the endof a cam shaft 12 for the exhaust valve 3. The pulley 12a is driven bythe pulley 11a via a belt. Accordingly, the intake valve 5 and theexhaust valve 3 are opened or closed at predetermined crank angles bythe crank shaft.

The cam shaft 11 is provided with a known variable valve timing controlmechanism which makes the pulley 11a rotate relative to the cam shaft11. For example, one variable valve timing control mechanism comprisesan intermediate gear which has outer and inner helical teeth, and whichconnects the cam shaft 11 with the pulley 11a. When the intermediategear is moved axially by oil pressure and the like, the cam shaft 11 isrotated relative to the pulley 11a. Thus, the variable valve timingmechanism can freely vary the valve overlap period by changing theopening time of the intake valve 5 in accordance with this relativerotation angle of the cam shaft 11.

Reference numeral 20 designates an electronic control unit (ECU) forcontrolling the valve overlap period via the variable valve timingmechanism, the amount of fuel injected via the fuel injector 7, and thedegree of opening of the ISC valve 10 via the step motor 10a. The ECU 20is constructed as a digital computer and includes a ROM (read onlymemory) 22, a RAM (random access memory) 23, a CPU (microprocessor,etc.) 24, an input port 25, and an output port 26. The ROM 22, the RAM23, the CPU 24, the input port 25, and the output port 26 areinterconnected by a bidirectional bus 21.

An engine speed sensor 31 which produces an output pulse representingthe engine speed is connected to the input port 25. A neutral switch 32for detecting a neutral condition of the transmission (not shown) and aclutch sensor 36 for detecting disengagement of the engine clutch (notshown) are also connected to the input port 25. In addition, an air flowmeter 33 produces an output voltage which is proportional to the amountof intake air fed into the engine cylinder, and this output voltage isinput into the input port 25 via an AD converter 27a. A throttle valvesensor 34 produces an output voltage which is proportional to the degreeof opening of the throttle valve 8, and this output voltage is inputinto the input port 25 via an AD converter 27b. A coolant temperaturesensor 35 produces an output voltage which is proportional to thetemperature of the cooling water of the engine as the enginetemperature, and this output voltage is input into the input port 25 viaan AD converter 27c. The output port 26 is connected to the variablevalve timing mechanism, the fuel injector 7, and the step motor 10a forthe ISC valve 10, via drive circuits 28a, 28b, 28c, respectively.

The ECU 20 controls the amount of fuel injected in accordance with theamount of intake air fed to the combustion chamber such that a desiredair-fuel ratio for the current engine operating condition is realized,and carries out a fuel cut operation during deceleration when thethrottle valve 8 is fully closed to save fuel and to prevent thecatalytic converter from over-heating, as is well known.

On the other hand, the ECU 20 also controls the valve overlap period andthe degree of opening of the ISC valve 10, according to a main routineshown in FIG. 2. The main routine is carried out, for example, at theend of every exhaust stroke in a certain cylinder, and is explained asfollows.

First, at step 11, the current engine speed (N), the current amount ofintake air fed into the engine cylinder (G), the current degree ofopening of the throttle valve (TA), and the current temperature of thecooling water (THW) are detected by the sensors 31, 33, 34, 35,respectively. Next, at step 12, a basic optimal value (AB) of the valveoverlap period during the current engine operating condition isdetermined from a map, which is provided for each temperature of thecooling water (THW), on the basis of the current engine load (G/N) (anamount of intake air fed into the engine cylinder per the unit enginespeed) and the current engine speed (N).

The map for a certain temperature of the cooling water is shown in FIG.9. In the map, a basic optimal value (ABnm) for each engine operatingcondition is written, such that the smaller the engine load (G/N)becomes, the smaller the basic optimal value (ABnm) becomes to realizestable combustion. This is because the negative pressure in an intakeport at the low engine load is relative high so that if the valveoverlap period is made long, the amount of back-flow exhaust gas becomeslarge. The larger the engine load (G/N) becomes, the larger the basicoptimal value (ABnm) becomes, to increase the trapping efficiency andthe scavenging efficiency, to take account of the current engine speed(N). On the other hand, the lower the temperature of the cooling waterbecomes, the smaller the corresponding basic optimal values (ABnm)generally become.

Next, the routine goes to step 13 and it is determined if the currentdegree of opening of the throttle valve (TA) is equal to or is smallerthan a predetermined maximum value (TA1) which indicates an idlecondition. When the result is negative, i.e., when the engine is not inan idle condition, the routine goes to step 14 and the actual valveoverlap period (A) at that time is set to the basic optimal value (AB).Next, the routine is stopped.

On the other hand, when the result at step 13 is positive, i.e., whenthe engine is in an idle condition, the routine goes to step 15 and thedegree of opening of the ISC valve 10 is determined from the map, shownin FIG. 10, on the basis of the temperature of the cooling water (THW)and the step motor 10a for the ISC valve 10 is actuated to realize thedetermined degree of opening.

In the map, the degree of opening of the ISC valve 10 is set such thatthe lower the temperature of the cooling water (THW) becomes, the largerthe degree of opening of the ISC valve 10 becomes, and the degree ofopening of the ISC valve 10. The minimum temperature of the coolingwater (THW1), which indicates that the engine warmed up, is set to 0%(fully closed), and the degree of opening of the ISC valve 10 at apredetermined temperature of the cooling water (THW2), which is lowerthan the minimum temperature (THW1) is set to 100% (fully open).Whereby, if the ISC valve 10 is moved by the step motor 10a according tothe map shown in FIG. 10, the lower the temperature of the cooling water(THW) becomes, the larger the amount of intake air passing through thebypass passage 9 becomes. Therefore, the amount of intake air fed to thecombustion chamber 1 increases and this causes an increase in the amountof fuel injected so that the idle engine speed becomes higher, and thuscombustion at this time becomes stable and an early warm-up can berealized.

Next, the routine goes to step 16 and it is determined if the ISC valve10 has moved according to the map shown in FIG. 10. When the result isnegative, the routine goes to step 14 and the actual valve overlapperiod (A) is set to the basic optimal value (AB) determined at step 12.However, when the result at step 16 is positive, i.e., when the ISCvalve 10 functions normally so that combustion becomes stable byincreasing an amount of intake air and fuel, the routine goes to step 17and a temporary valve overlap period (A') is calculated using thefollowing expression (1).

    A'=AB+k1*IA                                                (1)

Here, (k1) designates a predetermined coefficient. The temporary valveoverlap period (A') is increased from the basic optimal value (AB) atthis time, in accordance with the degree of opening of the ISC valve(IA).

Next, the routine goes to step 18 add a sub-routine for correcting thetemporary valve overlap period (A') is called, which will be explainedin detail. Thereafter, the routine goes to step 19 and the actual valveoverlap period (A) is set to the temporary valve overlap period (A').Next, the routine is stopped. In this manner, when the ISC valvefunctions normally in the idle condition, the actual valve overlapperiod (A) is made longer than the basic optimal value (AB). Therefore,although a back-flow of exhaust gas is caused, combustion at this timebecomes stable by increasing the amount of intake air and fuel fed tothe combustion chamber 1 and engine stall does not occur. Accordingly,the back-flow of exhaust gas is utilized as exhaust gas recirculation sothat the intake port 6 is heated by exhaust gas. Thus, the intake airtemperature is raised so that fuel is atomized favorably at this time,even though the engine has not warmed up, and better combustion can beobtained. Moreover, by such an exhaust gas recirculation, the amount ofNOx in the exhaust gas is reduced. This permits miniaturization of oromission of the heating means for heating the catalytic converter toactivate the catalyst when the engine has not warmed up.

FIG. 3 shows a first sub-routine used as the sub-routine called at step18 in the main routine. In the first sub-routine, at step 101, an upperlimit (dAmax) of the increasing value (k1*AI) of the valve overlapperiod is determined using the map shown in FIG. 11, on the basis of thecurrent temperature of the cooling water (THW). In the map, anincreasing value upper limit (dAmax) is set such that when thetemperature (THW) of the cooling water is higher than the ordinarytemperature (20 degrees C.), the lower the temperature of the coolingwater (THW) becomes, the larger the upper limit (dAmax) becomes. Whenthe temperature of the cooling water (THW) is lower than the ordinarytemperature, the lower the temperature (THW) of the cooling waterbecomes, the smaller the upper limit (dAmax) becomes. When thetemperature of the cooling water (THW) is the temperature (THW1), theupper limit value (dAmax) is set to "0".

Next, the routine goes to step 102 and it is determined if theincreasing value (k1*AI) in the main routine is equal to or is smallerthan the upper limit (dAmax) determined at step 101. When the result ispositive, the process returns to the main routine without changing theincreasing value (k1*AI). On the other hand, when the result at step 102is negative, the routine goes to step 103 and the increasing valve(k1*A1) is corrected to the upper limit (dAmax). Thereafter, the processreturns to the main routine and at step 19 in the main routine, theactual valve overlap period (A) is set to the temporary valve overlapperiod (A'=AB+dAmax) corrected by the first sub-routine. Accordingly,when the temperature of the cooling water (THW) is extremely low and theamount of intake air is increased considerably by the ISC valve 10, theactual valve overlap period (A) is not made long in accordance with theamount of intake air. As a result, when the temperature of the coolingwater (THW) is extremely low so that combustion is still relativelyunstable even if the engine speed is made higher, the amount ofback-flow exhaust gas does not increase so that engine stalling iscompletely prevented by the first sub-routine.

FIG. 4 shows a second sub-routine used as the sub-routine called at step18 in the main routine. In the second sub-routine, at step 201, thevalve overlap period (A) is set to the temporary valve overlap period(A') calculated at step 17 in the main routine and combustion in thecertain cylinder is carried out. Next, the routine goes to step 202 andan engine speed (N) during this combustion is detected by the enginespeed sensor 31. Next, the routine goes to step 203 and it is determinedif the engine speed (N) is equal to or is higher than a predeterminedlower limit engine speed (N1) which can maintain an idle condition.

When the result is positive, the process returns to the main routinewithout correction of the temporary valve overlap period (A') and thisvalve overlap period (A') is used as the actual valve overlap period (A)in other cylinders until a new temporary valve overlap period (A') iscalculated. On the other hand, when the result at step 203 is negative,the routine goes to step 204 and the temporary valve overlap period (A')is corrected to the basic optimal value (AB). Next, the process returnsto the main routine. Accordingly, in the case that the temperature ofthe engine oil is low so that the coefficient of viscosity of the engineoil is relative large and thus the engine speed is made low by the largecoefficient of viscosity of the engine oil, the actual valve overlapperiod (A) in other cylinders is not increased from the basic optimalvalue (AB) so that the engine speed becomes higher than the lower limitengine speed (N1) and thus engine stall is prevented.

FIG. 5 shows a third sub-routine used as the sub-routine called at step18 in the main routine. In the third sub-routine, first, at step 301,the valve overlap period (A) is set to the temporary valve overlapperiod (A') calculated at step 17 in the main routine and combustion inthe certain cylinder is carried out. Next, the routine goes to step 302and the engine speed (N) during this combustion is detected by theengine speed sensor 31. Next, the routine goes to step 303 and adifference (dN) between the engine speed (N) and a target engine speed(N') at the current temperature of the cooling water (THW) iscalculated. Next, the routine goes to step 304 and a coefficient (k2) isdetermined by a map shown in FIG. 12, on the basis of the difference(dN) thereafter the temporary valve overlap period (A') is multiplied bythe coefficient (k2) and is made a new temporary valve overlap period(A'). Next, the process returns to the main routine.

In the map, the coefficient (k2) is set such that when the difference(dN) is "0", the coefficient (k2) is "1", and the larger the difference(dN) becomes, the larger the coefficient (k2) becomes. Accordingly, ifthe current engine speed (N) is higher or lower than the target enginespeed (N') at this time, the actual valve overlap period is made largeror smaller in accordance with the difference (dN) so that the amount ofexhaust gas recirculation is adjusted so as to realize the target enginespeed (N'). Due to the correction of the valve overlap period by thethird sub-routine, in addition to the effects resulting from increasingthe valve overlap period as above mentioned, stable combustion in anidle condition can be certainly realized. For example, when the ISCvalve 10 is fully open or fully closed and the engine speed (N) is loweror higher than the target engine speed (N'), the idle speed can not becontrolled by the ISC valve 10. In this case, the third sub-routine isespecially effective.

FIG. 6 shows a fourth sub-routine used as the sub-routine called at step18 in the main routine. In the fourth sub-routine, first, at step 401,it is determined if the engine transmission is in neutral, by using theneutral switch 32. When the result is negative, the routine goes to step402 and it is determined if the engine clutch is disengaged, by usingclutch sensor 36. When both results are negative, i.e., the vehicle isrunning normally, the process returns to the main routine withoutcorrection of the temporary valve overlap period (A'). On the otherhand, when the result at step 401 or 402 is positive, the routine goesto step 403, and the temporary valve overlap period (A') is added to apredetermined value (a) and is made the new temporary valve overlapperiod (A'). Next, the process returns to the main routine.

When the engine transmission is in neutral or the clutch is disengaged,i.e., when the engine is disengaged apart from the vehicle body and thevehicle is stopped, the engine vibration is not transmitted to thevehicle body via the engine drive shaft. Accordingly, in this case, ifthe actual valve overlap period (A) is made larger by the fourthsub-routine, in spite of the deterioration of combustion and the enginevibration, the vehicle body is hardly vibrated by the engine. In result,the amount of NOx in exhaust gas can be reduced and better heatingintake air can be realized by an increase of the amount of the back-flowexhaust gas. Note that when the vehicle has an automatic transmission,step 402 in the fourth sub-routine is omitted because the vehicle is notprovided with the clutch.

FIG. 7 shows a fifth sub-routine used as the sub-routine called at step18 in the main routine. In the fifth sub-routine, at step 501, it isdetermined if a fuel cut operation during deceleration to save fuel andprevent excess heating of the catalytic converter is carried out. Whenthe result is negative, the process returns to the main routine withoutcorrection of the temporary valve overlap period (A'). On the otherhand, when the result at step 501 is positive, the routine goes to step502 and the temporary valve overlap period (A') is added to apredetermined value (b) and is made the new temporary valve overlapperiod (A'). Next, the process returns to the main routine.

During the fuel cut operation, combustion is not carried out and no fuelis injected. Therefore, the valve overlap period can be set to anyvalue. Once the actual valve overlap period (A) is made larger by thefifth sub-routine, the back-flow amount of air heated in the combustionchamber 1 becomes large so that good heating of intake air is realizedwhen the fuel injection starts again. In the fifth sub-routine, anincreasing value (b) of the valve overlap period at step 502 is aconstant. However, it may be a variable which is determined from the mapshown in FIG. 13, on the basis of the engine speed (N) immediatelybefore the fuel cut operation starts. In the map, the variable is setsuch that the higher the engine speed is, the larger the variablebecomes. The higher the engine speed immediately before the fuel cutoperation starts is, the higher the temperature in the combustionchamber 1 becomes. Accordingly, the actual valve overlap period (A) ismade large in accordance with the engine speed immediately before thefuel cut operation starts by the fifth sub-routine so that heat of thecombustion chamber 1 can be used effectively to heat the intake air.

FIG. 8 shows a sixth sub-routine used as the sub-routine called at step18 in the main routine. In the sixth sub-routine, first, at step 601, itis determined if the fuel cut operation during deceleration, to savefuel and prevent excess heating of the catalytic converter, is carriedout. When the result is negative, the routine goes to step 602, andthereafter the routine goes to step 603 and a count value (n) is resetto "0". Next, the process returns to the main routine without correctingthe temporary valve overlap period (A') determined at step 17 in themain routine. The process at step 602 is a calculation of an extraamount (dTAU) of fuel injected initially when the fuel injection startsagain. The count value (n) remains "0" when the fuel cut operation isnot carried out, so that the extra amount (dTAU) remains at "0". Thecalculation is explained in detail later.

On the other hand, when the result at step 601 is positive, i.e., whenthe fuel cut operation is carried out, the routine goes to step 604 andthe count value (n) is increased by "1". Next, the routine goes to step605 and it is determined if the count value (n) is "1". The result ispositive only immediately after the fuel cut operation starts and theroutine goes to step 606. At step 606, the temperature in the combustionchamber 1 (T) is estimated. For this estimation, the amount of fuelinjected and the temperature of the cooling water, immediately beforethe fuel cut starts can be utilized. It is understood that thetemperature in the combustion chamber 1 (T) is high when the amount offuel is large and the temperature of the cooling water is high. On theother hand, the temperature in the combustion chamber 1 (T) may bedetected directly.

Next, the routine goes to step 607, and an increasing value of the valveoverlap period (c) is determined from the map shown in FIG. 14, on thebasis of the temperature in the combustion chamber 1 (T). In the map, anincreasing value of the valve overlap period (c) is set such that thehigher the temperature in the combustion chamber 1 (T) becomes, thelarger the increasing value (c) becomes to effectively utilize the heatof the combustion chamber 1. Next, the routine goes to step 608 and anew increasing value (d) is calculated using the following expression(2), on the basis of the increasing value (c) determined at step 607.

    d=c-q(n-1)                                                 (2)

Since the count value (n) is "1" at present, the new increasing value(d) is equal to the increasing value (c). However, if the fuel cutoperation continues, the count value (n) is increased and the processesat steps 606 and 607 are omitted and the new increasing value (d) isdecreased, by a predetermined value (q), gradually. Next, the routinegoes to step 609 and it is determined if the new increasing value (d) issmaller than "0". Only when the result is positive, the routine goes tostep 610 and the new increasing value (d) is set to "0". Next, theroutine goes to step 611, and the temporary valve overlap period (A') isadded to the increasing value (d) and is made the new temporary valveoverlap period (A'). Next, the process returns to the main routine.

According to the sixth sub-routine, the actual valve overlap period (A)during a fuel cut operation is increased by the increasing value (c)determined on the basis of the temperature in the combustion chamber 1(T) and thereafter is decreased by a predetermined value (q), as shownin FIG. 15. The temperature in the combustion chamber 1 (T) dropsgradually due to the repeated back flow of intake air so as to achievethe effect that heat to the intake port can not improved even if thevalve overlap period remains long. Accordingly, the actual valve overlapperiod (A) is gradually decreased by the sixth sub-routine so that thedifference between the valve overlap periods before and after the fuelinjection starts can be made small and thus the engine torque variationat this time can be reduced.

Moreover, in the sixth sub-routine, when the fuel injection starts againafter a fuel cut operation, the result at step 601 is negative and theroutine goes to step 602. Here, the count value (n) is not "0" and is acertain value so that an extra amount (dTAU) added to a required amountof fuel injected, determined on the basis of the engine speed, theengine load, and the engine temperature is calculated according to thefollowing expression (3). ##EQU1##

In the expression (3), ##EQU2## represents the hatched area in the timechart shown in FIG. 15. The extra amount (dTAU) equals is this areamultiplied by a predetermined coefficient (k3). Accordingly, the longerthe fuel cut period becomes or the larger the increasing value (c) ofthe valve overlap period becomes, the larger the extra amount (dTAU)calculated becomes. When the fuel cut period is long or when the valveoverlap period is long, fuel stuck to the inside wall of the intake portevaporates considerably due to the sufficient heating of the intake portbefore fuel injection starts. The extra amount (dTAU) surely compensatesfor the amount of fuel evaporated from the inside wall of the intakeport so that a required amount of fuel can be fed to the combustionchamber 1 when the fuel injection starts. When the normal fuel injectionis carried out during an intake stroke, the extra amount (dTAU) of fuelmay be injected separately during an exhaust stroke.

In the previous embodiments, the variable valve timing mechanism canvary the valve overlap period continuously. However, it is clear thatthe present invention can be realized by a variable valve timingmechanism which varies the valve overlap period in stages.

In the main routine, any of the sixth sub-routines can be carried out.More than one of the sub-routines may be carried out in series.Moreover, the fifth or the sixth sub-routine is carried out during anidle condition. However, it should be understood that these twosub-routines are effective when the engine has not warmed up, in spiteof the degree of opening of the throttle valve.

Although the invention has been described with reference to specificembodiments thereof, it should be apparent that numerous modificationscan be made thereto by those skilled in the art, without departing fromthe basic concept and scope of the invention.

I claim:
 1. A valve timing control device for an internal combustionengine comprising:a variable valve timing mechanism capable of varying avalve overlap period continuously or in stages; increasing means capableof increasing an amount of intake air, at least during an engine idlespeed condition, said increasing means increasing the amount of intakeair continuously or in stages; determination means for determining anoptimal value of the valve overlap period during a current engineoperating condition, on the basis of current engine speed, load, andtemperature; first control means for controlling said variable valvetiming mechanism such that the valve overlap period becomes larger thansaid optimal value during said engine idle speed condition when theengine has not warmed up, said first control means controlling saidvariable valve timing mechanism such that the larger said amount ofintake air becomes, the larger the valve overlap period becomes duringsaid engine idle speed condition when the engine has not warmed up; andsecond control means for controlling said increasing means such that theamount of intake air is increased during said engine idle speedcondition when the engine has not warmed up, said second control meanscontrolling said increasing means such that the lower the enginetemperature becomes, the larger the amount of intake air becomes duringsaid engine idle speed condition when the engine has not warmed up.
 2. Avalve timing control device according to claim 1, wherein a targetengine speed is determined for each engine temperature, said valvetiming control device further comprising correction means for correctingthe valve overlap period such that the larger a difference between saidtarget engine speed and the current engine speed becomes, the smallerthe valve overlap period becomes when said current engine speed is lowerthan said target engine speed.
 3. A valve timing control deviceaccording to claim 1, wherein said increasing means includes a bypasspassage which bypasses a throttle valve, and a valve arranged in saidbypass passage.
 4. A valve timing control device according to claim 1,further comprising:detection means for detecting when a transmission isin a neutral condition; and correction measn for increasing the valveoverlap period when the transmission is in the neutral condition.
 5. Avalve timing control device according to claim 4, wherein said detectionmeans comprises a neutral switch for detecting that the transmission isin the neutral condition.
 6. A valve timing control device according toclaim 4, wherein said detection means further comprises means fordetecting a clutch disengagement condition.
 7. A valve timing controldevice according to claim 1, further comprising stop means forinhibiting control of said first control means when a current enginespeed is lower than a predetermined lower limit engine speed necessaryto maintain an engine idle speed condition.